The present invention relates to a photoconductive drum device for copying machines of the electrostatic record type or the like which is characterized by a heater accommodated in the photoconductive drum for maintaining the drum at a constant proper temperature.
Electrostatic record apparatus such as electrostatic copying machines comprise a photoconductive drum having a photoconductive layer over the surface. The drum is sensitized by a main charger while being rotated and thereafter exposed to an optical image to thereby form an electrostatic latent image on the photoconductive layer. The latent image is developed by a developing unit to a toner image, which is then transferred onto copy paper. The photoconductive material for use in such copying machines or the like is generally susceptible to the influence of temperature, so that it has been conventional practice to maintain the photoconductive layer at a constant temperature by a heater accommodated inside the drum.
FIGS. 11 (A) and (B) show one such conventional temperature maintaining heater. The illustrated heater 101 comprises a rectangular base plate 105 made of a resilient material such as stainless steel and bent into a hollow cylindrical form so as to be accommodated in a photoconductive drum 102. The base plate 105 is provided with a heat generator 106, for example, of nichrome wire extending in a zigzag pattern and fixed to one surface of the base plate. However, with the heater 101 wherein the base plate 105 is originally rectangular, the base plate 105 is liable to deform and scratch the edge of the drum 102 owing to its restoring force when the heater is inserted into the drum 102 while being bent into a cylindrical form against the restoring force. The heater is further very cumbersome to insert into the drum because the opposed longitudinal side portions thereof are displaced from each other and need to be correctly positioned after insertion. Additionally, when the heater is accommodated in the drum 102, it is difficult to hold the rectangular base plate 105 in the cylindrical bent form accurately along the inner surface of the drum 102. In fact, it is difficult to intimately fit the base plate 105 to the drum inner surface especially at the longitudinal side portions as seen in FIG. 11 (B). To assure intimate fitting contact, there arises a need to use fasteners (not shown) for pressing the base plate 105 against the drum inner surface. This makes the insertion procedure all the more cumbersome. When the heater 101 is accommodated in the drum 102, the straight portions of the heat generator 106 attached to the base plate 105 are arranged in parallel with the axis of the drum 102 and spaced at a distance circumferentially of the drum 102. Temperature irregularities are therefore liable to occur circumferentially of the drum, possibly resulting in irregularities in the transferred image. The temperature irregularities will be diminished if the portions of the heat generator 106 are arranged closely, but this renders the heater disadvantageous in respect of production process and cost.
It is required to control the temperature of the photoconductive drum with high precision when the photoconductive material used has temperature dependence. Devices are known which comprise a temperature maintaining heater for heating the photoconductive drum, a temperature sensor, such as a thermistor, for detecting the temperature of the drum, and a control unit for controlling the amount of current to be passed through the heater in response to the temperature detection signal from the sensor. With the conventional device of this type, the temperature sensor is disposed at the longitudinal middle portion of the drum or an end portion thereof, but this leads to difficulty in properly controlling the overall temperature of the drum. When the drum is checked for longitudinal temperature distribution with the heater energized, it is seen that the drum has a low temperature at its opposite ends due to the release of heat from the flanges and also at the longitudinal middle portion where the copy paper removes heat during copying operation, the drum having the highest temperature at other portions. Accordingly, if the temperature sensor is disposed at the middle or end portion of the drum for controlling the temperature, the highest temperature portion becomes overheated resulting in the likelihood that the drum will deteriorate thermally or the toner supplied to the drum will be degraded.